1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display device, and particularly to a method for manufacturing a liquid crystal display device incorporating therein a plastic substrate as a substrate thereof.
2. Description of the Related Art
A plastic substrate has been considered a promising substrate material for compact, thin and low cost liquid crystal display device. Japanese Patent Application No. 13(2001)-125082 discloses a method for manufacturing a liquid crystal panel employing therein such a plastic substrate. The plastic substrate employed in the liquid crystal display device disclosed in the publication will be briefly explained with reference to FIGS. 1A through 2B.
As shown in FIG. 1A, first, a thermally activatable adhesive 212 attached with a removable film 211 is pressed against and attached to a support substrate 203 using a roller. Then, as shown in FIG. 1B, the removable film 211 is peeled off from the thermally activatable adhesive 212 attached to the support substrate 203. Furthermore, as shown in FIG. 1C, a plastic substrate 201 having a transparent electrode 213 formed on the surface of the substrate is adhesively attached using a roller 215 to the support substrate 203 via the thermally activatable adhesive 212.
The inventors of this application identified the following problems occurred when the plastic substrate fabricated as described above is handled in actual manufacturing steps.
Referring to a side view shown in FIG. 2A, air 214 enters between the thermally activatable adhesive 212 and the plastic substrate 201. Even when trying to pull out the air left between the thermally activatable adhesive 212 and the plastic substrate 201, the adhesive 212 blocks a passage along which the air escapes to the outside and therefore, air is always left between the adhesive 212 and substrate 201. Thereafter, the plastic substrate 201 supported by the support substrate 203 is rinsed with pure water and chemical solution, and then dried at a temperature of 130–170°C. Then, an alignment material, heated at low processing temperatures, is applied to the transparent electrode 213 on the plastic substrate 201 by printing techniques and heated at a temperature of 80–180°C. Subsequently, the heated alignment material is rubbed to form an alignment film and rinsed with pure water, and then dried at a temperature of 130–170°C.
The plastic substrate 201, having been subjected to the aforementioned heat treatment, is deformed because the air 214 thermally expands and grows into a large bubble. In some cases, the plastic substrate 201 is broken by expansion of bubble.
Moreover, as shown in FIG. 2B, an adhesive sealing material 204, provided as an thermally cured adhesive and having a specific patterns is formed on the surface (on the side of an alignment film) of the plastic substrate 201, which is supported by the support substrate 203, by screen printing techniques or dispensing techniques. Then, a TFT substrate 251 having spacers (not shown) dispersed on the surface thereof and the plastic substrate 201 supported by the support substrate 203 are attached together along the edge with the adhesive sealing material 204 and the two substrates are pressed against each other, and further, heated for 1 to 2 hours at a temperature of 120–160°C., resulting in their attachment.
The two substrates, attached to each other as described above, encounter a situation in which a cell gap between the two substrates becomes non-uniform, i.e., the adhesive sealing material is displaced from its original position or the plastic substrate is subject to breakage. This is because variations in the extent to which the thermally activatable adhesive 212 is adhesively attached to the plastic substrate occur and/or the air 214 left between the thermally activatable adhesive 212 and the plastic substrate 201 and subjected to heat treatments in the step of sintering the sealing material, expands resulting in the deformation and/or flexure of plastic substrate. Furthermore, the fact that the amount of expansion or shrinkage of plastic substrate 201 is larger than that of the support substrate 203 at high processing temperatures (i.e., during sintering of sealing material) also contributes to deformation and/or flexure of plastic substrate. In other words, the adhesion force of the thermally activatable adhesive 212 provided to the plastic substrate 201 cannot suppress expansion or shrinkage of plastic substrate.
Moreover, since the plastic substrate, constructed as described above, has the thermally activatable adhesive attached thereto, it needs to be processed through time-consuming steps including an attachment step and a peeling step. Thereby, the number of process steps is increased.